In the manufacture of cathode ray tubes (CRTs) it is necessary to provide a coating of conductive material, e.g. aluminum, on the internal surface of the faceplate and the forward part of the funnel, in order to establish an electric field within the CRT for accelerating electrons from the electron gun, disposed in the neck of the CRT, to the faceplate.
Conventionally, the internal surface of a CRT is aluminized by inserting a braided tungsten filament, having one or more foils of aluminum draped over it, into the interior of the CRT. The filament is connected to a source of electrical energy, and the resultant heating of the filament causes the aluminum to boil, establishing an aluminum vapor inside the CRT. Vapor condenses on the internal surface of the CRT, providing the desired aluminum coating. Unfortunately, this process is expensive, since the tungsten filament must be replaced quite frequently, and it is also inconsistent, since the molten aluminum wicks along the filament so that the final location from which the aluminum evaporates is not the same as the initial location of the aluminum foils. In addition, the molten aluminum alloys with the tungsten of the filament, and causes the filament to be eroded and become brittle. The increased brittleness of the filament increases the risk of breakage, because there is necessarily a physcial impact to the filament each time a new foil is applied to the filament.
Erosion of the filament is a more serious problem than brittleness. Erosion does not occur uniformly along the length of the filament, but creates regions of reduced cross-section area along the filament. These regions become much hotter than other regions of the filament when the filament is heated, accelerating the erosion and necking of the filament. If the aluminum foil is not fully evaporated in each cycle, the residue builds up on the cooler regions of the filament, requiring a still higher current to heat these regions sufficiently to evaporate the aluminum, accelerating the erosion of the hotter regions. The filament ultimately fails at the eroded region.
Working of the filament clamps during heating and cooling of the filament also stresses the filament, making it more likely to break.
Electron beam (E-beam) apparatus is used for aluminizing objects in the manufacture of, e.g., hybrid circuits, integrated circuits and thin film devices. One conventional E-beam apparatus comprises a graphite crucible in which the aluminum is placed, and an E-beam source, which may be a scanning E-beam source. The crucible is biased at a positive potential, and the E-beam source is disposed to direct the E-beam downwardly into the aluminum in the crucible. The crucible, E-beam source and workpiece to be aluminized are all placed in an evacuated enclosure. The electrons impinge on the aluminum in the crucible, heating the aluminum and vaporizing it. Aluminum vapor condenses on the workpiece, providing the desired aluminum coating. E-beam aluminizing, as conventionally practiced, cannot be applied to CRTs, which are aluminized with the faceplate disposed generally horizontal and above the funnel, because it would require the E-beam source to be located inside the tube. It would be difficult, if not impossible, to provide an E-beam source small enough to fit inside a CRT, immediately beneath the faceplate, because the source would have to be inserted into the tube and removed from the tube by way of the neck of the tube.